This invention is directed to an improved fault monitoring circuit for a reverse cycle air-conditioner, commonly referred to as a heat pump. More specifically, this invention is directed toward a fault detection circuit for a heat pump system which incorporates auxiliary resistance heaters for providing additional heat when the outside temperature is below the "balance point" of the heat pump system. Such circuitry is to be utilized for detecting a loss of efficiency in the heat pump system whereby the resistance heaters are prematurely activated.
An electrically operated heat pump provides the means for reducing the amount of energy utilized in heating a building. The electric energy that operates the heat pump provides only part of the heat output, the remainder is obtained from an external source such as air or water. The ratio of the heat output to electrical input (in equivalent units) is the co-efficient of performance (COP) of the heat pump and can be equal to 2 or higher. With recent increases in the cost of other sources of heat, the heat pump has become a competitive heating system.
However, an inherent characteristic of the heat pump when operated as a heat source is that as the outside air temperature goes down, the heat output from the heat pump system is reduced. There is an outdoor temperature at which the heat pump is unable to supply the heating requirements for the building it is servicing, this is known as the "balance point".
In order that indoor temperatures will not reach uncomfortable levels when the outdoor temperature is below the balance point, electrical resistance heaters are commonly provided within the heat pump system to supplement heat pump output. However, such heaters have a COP of only 1, and thus are much more expensive to operate than the heat pump. It is desirable therefore that the resistance heater operation is kept to a minimum.
Several stages of resistance heat are typically provided in heat pump systems, which stages are energized sequentially to prevent large surges of load current on the electrical system and to avoid using more such heat than absolutely necessary. The first stage of such resistance heating is typically energized from an indoor thermostat, when the indoor temperature falls about 2.degree. below the thermostat set point, indicating that the heat pump is unable to supply sufficient heat to maintain the desired indoor temperature. Subsequent stages of resistance heating are controlled by varying methods, outdoor thermostats and time delays being among them.
For a heat pump that is operating as designed, the resistance heaters should not be energized as long as the outdoor temperature is above the balance point. It is possible, however, for the heat pump to lose part of its heating capacity through a malfunction such as a minor refrigerant leak, so that it is not operating at peak efficiency and cannot supply sufficient heat at the balance point or possibly some degrees above it. If this occurs, the resistance heaters will be energized by the indoor thermostat. The building occupant more than likely will not be aware that this is happening, as a comfortable indoor temperature is being maintained. He will become aware, later, when he receives his electric bill, such bill normally covering a month or more of such operation, because the resistance heat will have consumed much more electricity.
It is desirable to provide a device which will alert the building occupant to the existence of a loss of system efficiency and premature activation of resistance heaters in the heat pump so that he may provide corrective maintenance promptly and therefore minimize electrical power consumption. No such system degradation device is currently in the marketplace.
An object of this invention is to provide a heat pump system with a balance point thermostat, said thermostat being set at the empirical, or alternatively, the mathematically calculated, balance point outdoor temperature for a heat pump system.
A second object of this invention is to provide a monitoring circuit for detecting when the heat pump is in the heating mode, and the auxiliary resistance heater has been prematurely activated.
A further object of this invention is to provide a detectable signal, announcing that the auxiliary heaters of the heat pump system have been prematurely activated indicating loss in efficiency of the system.
An even further object of this invention is to provide this monitoring circuit for electrically determining when the heat pump system is in the heating mode, the outdoor temperature is above the balance point threshold temperature, and the resistance heater is activated, to provide an alarm signal indicating loss of efficiency.